This invention relates to an adjustable hydraulic shock absorber and system which utilize a fluid powered actuator to adjust a damping characteristic of the shock absorber
Our previous U.S. Pat. No. 4,838,394 discloses an adjustable shock absorber and system which utilize fluid powered actuators such as pistons to control damping characteristics of the shock absorber. The shock absorbers disclosed in the '394 patent use three different devices for energy dissipation: (1) a low speed bleed orifice defined by the tapered slot 96 and the valve plate 92; (2) a spring loaded blow off valve comprising the valve plate 92 and the spring 100; and (3) a high speed restriction comprising the fixed restricting orifice defined by the plate 88. The low speed bleed orifice is integrated with the blow off valve, and the maximum pressure drop across the low speed bleed orifice is limited to the pressure at which the blow off valve opens. The high speed restriction provided by the plate 88 is in series with the blow off valve and is therefore always operative; however, its contribution to energy dissipation is negligible at low speeds.
The resulting damping forces provided by these three devices are shown in FIGS. 7 and 8 of the '394 patent. The steep rise at low speeds is attributable to the low speed bleed orifice, the knee of the curve is attributable to lifting of the blow off valve, and the increase in damping force with piston speed for piston speeds higher than the knee of the curve is attributable to the high speed restriction. Of course, there is also a constant dry friction force that is generally minimized in good designs because it produces a harsh ride.
At low speeds, the total damping force is the sum of friction, low speed bleed orifice forces and high speed restriction forces. After blow off, it is the sum of friction, blow off forces and high speed restriction forces.
In the system illustrated in the '394 patent all four shock absorbers are controlled by a single static pressure source 26 (arbitrarily defined as one having full scale variations in control pressure with a frequency no greater than one second). Alternately, as discussed in column 9, two separate control systems can be provided, each with its own pressure source to allow the driver to control the front and rear shock absorbers independently of one another. The '394 patent also suggests that the adjustable pressure reducing valve can be controlled by an automatic control system to prevent a vehicle from diving on deceleration or squatting on acceleration. Such a system would require a response time of about 300 ms.
Another approach to adjustable shock absorber design is to add a parallel bleed orifice that it is always effective. If this parallel bleed orifice is made variable, the damping force can be made variable. In general, the larger the parallel orifice, the lower the damping force.
Many prior art dampers, including adjustable dampers, use these three basic mechanisms. Many adjustable dampers use an electric motor or a solenoid powered valve to open a parallel orifice to the main damper valve. The '394 patent adjusts both the orifice and the blow off points, to get greater variation than that obtained merely by changing parallel orifice sizes, but the '394 patent design still uses the basic three stage valving technique. One disadvantage to this approach is that the damping force remains a function of the piston velocity of the damper. This function can be varied in adjustable dampers, but for a given setting of the adjustment the damping force still increases when piston velocity increases.
"A novel valve for semi-active vehicle suspension systems" by G. A. Parker, et al. (1988) discloses a damper system that uses an electrical feedback system to control damping pressure and therefore damping force. An electromagnetic valve at the damper modulates the damping pressure of the working hydraulic fluid in response to electrical control signals. This approach differs significantly from that of the '394 patent in that there is no hydraulic control circuit and no fluid powered actuator that adjusts the damping characteristics of the damper.
It is an object of this invention to provide an adjustable damper that utilizes a fluid powered actuator responsive to fluid pressure in a control circuit, which is higher in response speed than the system disclosed in the '394 patent, and is well suited for use in active damping systems.